1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device. More specifically, the present invention relates to an integrated circuit chip in which initialization is performed by using a ROM Fuse.
2. Description of the Related Art
Integrated circuit chips (IC chips) such as NAND type EEPROM having a electrically programmable nonvolatile semiconductor memory have increasingly employed a ROM fuse for an initial setup operation (for example, see Japanese Patent Application No. 2001-176290). The ROM Fuse is a predetermined data region 101a on a memory cell array (or dedicated memory) 101, in which various initial setup values required for initializing an IC chip are stored, as shown in FIG. 12. The various initial setup values stored in the predetermined-data region 101a are automatically read by a sense amplifier (S/A) 105 immediately when the power-on of the IC chip is detected by a control circuit 103.
The various initial setup values read out from a ROM Fuse are then transferred to individual registers (data latch circuits) 107a, 107b, and 107c and stored therein. The various initial setup values stored in individual registers 107a, 107b, and 107c are respectively sent to the corresponding circuits. For example, the initial setup values for setting up a write voltage and erase voltage stored in the register 107c are sent to a high-voltage generation circuit 109. In this manner, the initial setup operation (initialization) of the IC chip is performed. The ROM Fuse is advantageous since the design can be made with a high degree of freedom compared to conventional fuses such as a laser-melt type fuse and the cost for a test run can be saved. Accordingly, the demand for the ROM Fuse has recently increased.
However, since initial setup values are actually written in memory cells in the form of data in a ROM Fuse, the reliability of data is lower than those of conventional fuses. To overcome the low reliability, the following method has been contemplated.
For example, not a single set of initial setup data but several sets thereof are stored in a ROM Fuse and the initial setup data of individual sets are compared at power-on. If this method is employed, the reliability of data can be improved.
As explained above, the reliability of data in a ROM Fuse can be improved by comparing several sets of initial setup data. However, there is a possibility that data gets garbled. It is also considered that all initial setup data of each set may be put in a FAIL status. More specifically, in the initial setup operation, since data is read out when power is turned on, the readout data greatly varies depending upon the state of power. Consequently, it is sometimes impossible to correctly read out the initial setup data. In such a case, effective initial setup data cannot be set during the initial setup operation. This inevitably means the initialization of the IC chip is incomplete. For example, defective cells have not been replaced for redundancy and the trimming of power has not been performed.
When the initialization is not effectively made not only a normal memory operation of an IC chip cannot be ensured but also disappearance of valuable cell data and destruction of memory cells themselves may occur. Accordingly, it has been desired to develop an effective means for protecting an IC chip from the disappearance of cell-data and so forth even when the initial setup operation is not successfully completed by failure of the initial setup operation.